Missing mesh option creating WiFi network

I have two Zyxel Ax1800 access points and a Zyxel EX3301-T0 router. I want to create a mesh network and have followed the set up video but in the WiFi network section of the Nebula App the set up videos show a slider ‘Smart Mesh’ but it doesn’t appear in my App. I can’t work out why and it’s preventing me connecting one access point to the other as a Mesh. Any ideas??